1. Field of the Invention
The present invention relates to an optical fiber module and an optical device.
2. Description of the Related Art
Optical fibers are used in, for example, optical communication systems for various applications, and various optical characteristics of the optical fibers are required depending on the application.
For example, a highly nonlinear fiber (HNLF) having high nonlinearity has been developed, and accordingly, a signal processing based on nonlinear optical effects that occur in the HNLF is put into practice. The HNLF is used for wavelength conversion based on the four-wave mixing (FWM), the optical pulse compression based on the self-phase modulation (SPM), the optical soliton transmission, the waveform shaping, supercontinuum light sources, and the like. Such HNLFs need to have the most suitable chromatic dispersion for the nonlinear optical effect. For example, in the case of the wavelength conversion based on the FWM, a broadband wavelength conversion can be realized with high efficiency by precisely matching the zero-dispersion wavelength of the HNLF to a predetermined wavelength (see, for example, Japanese Patent Application Laid-open No. 2005-301009).
On the other hand, when a signal light is input to an optical fiber, an acoustic wave in the optical fiber scatters a part of the signal light to generate a scattered light. This scattered light is called the Brillouin-scattered light, which has a peak at an optical frequency lower than that of the signal light by the frequency of the acoustic wave, and which propagates in the backward direction opposite to the direction of transmission of the signal light. The difference between the optical frequency of the signal light and the peak optical frequency is called Brillouin frequency shift (BFS) unique to an individual type of the optical fiber. A monitoring system that detects a failure location in a line of a fiber to the home (FTTH) system by using a plurality of optical fibers designed to have different BFS is disclosed in Nazuki Honda, et al., “Newly developed optical fiber line testing system employing termination cable with individual Brillouin frequency shift”, Proceedings of the Society Conference of IEICE, B-10-10, 2005.
When the optical intensity of an incident light exceeds a predetermined threshold, the Brillouin-scattered light causes a stimulated scattering to generate the stimulated Brillouin-scattered (SBS) light. The SBS threshold generally becomes lower as the optical fiber gets longer. When the SBS light is generated, no matter how the optical intensity of the incident light is increased, energy of the incident light is consumed by the SBS light. Therefore, for example, in the case of the wavelength conversion, an increase of the optical intensity of the incident light cannot increase the optical intensity of the light that contributes to generation of the FWM, disadvantageously resulting in low efficiency of the wavelength conversion.
Various methods for increasing the SBS threshold by changing characteristics of an optical fiber, such as the chromatic dispersion and the attenuation loss, in the longitudinal direction of the optical fiber by changing the core diameter or the amount of dopant doped into the core are disclosed in Japanese Patent No. 2584151, Japanese Patent No. 2,753,426, and Japanese Patent No. 3580640. According to the methods, because the BFS changes in the longitudinal direction of the optical fiber, the SBS light is hardly generated, which means that the SBS threshold becomes high.
The optical characteristics of an optical fiber, such as the chromatic dispersion and the BFS, are determined by materials and refractive index profile of the optical fiber.
The optical fiber is generally manufactured by drawing an optical fiber preform to a target cladding diameter (outer diameter) so that the drawn optical fiber has desired optical characteristics. For this reason, most of the optical characteristics are determined when the optical fiber is manufactured. The optical fiber thus manufactured to have the desired optical characteristics is usually incorporated into an optical fiber module or an optical device.
However, the optical characteristics of the manufactured optical fiber show natural fluctuations due to a design error of the target refractive index profile, a fluctuation of the target refractive index profile in the longitudinal direction of the optical fiber, and the like. For this reason, it is hard to obtain an optical fiber module or an optical device having the desired optical characteristics.